The effects of rainfall on mass transfer across the air-water interface and turbulent structure were investigated through laboratory experiments in a turbulent open-channel flow. The air-water CO2 transfer coefficient due to raindrops impinging on the air-water surface and turbulence quantities in an open-channel flow were measured. The results show that rainfall enhances turbulent mixing near the free surface on the liquid side, and the enhancement results in increase of CO2 transfer across the air-water interface. The mass transfer coefficient on the liquid side is well correlated with the mean vertical momentum flux of rainfall (MF), but it is not well correlated with the mean kinetic energy flux of raindrops impinging on the unit area of the air-water interface (KEF), as has been proposed in previous studies. The maximum value of the mass transfer coefficient obtained in the present rain experiments corresponds to that observed in oceans with high wind speeds. This suggests that it is of great importance to consider the effects of rainfall in precisely estimating the local air-sea CO2 exchange rate.
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